The term “wettability” describes the tendency of a fluid to spread on, and adhere to, a solid surface in the presence of other immiscible fluids, i.e., to “wet” the surface. Within a subterranean hydrocarbon reservoir, wettability influences relative permeabilities, residual oil saturation and capillary pressure. Consequently, it becomes important to be able to estimate reservoir wettability.
Laboratory analysis of core samples can estimate reservoir wettability. However, this is both costly and time consuming. For example, several weeks may be required to perform a wettability test using the Amott or USBM technique, and practical constraints limit the extent and number of cores that can be analyzed. In addition, the reservoir wettability conditions can change by the time samples undergo laboratory analysis, thereby rendering the results inaccurate. Therefore, it is beneficial to determine rock wettability in-situ in its original unaltered state.
Bona, N., and Capacciolo, S., 2001, “Electrical Measurements in the 100 Hz to 10 GHz Frequency Range for Efficient Rock Wettability Determination”, SPE Journal, March, pp. 80-88 describes a technique that identifies oil-wet rock by comparing the high- and the low-frequency conductivity of the rock. If the high-frequency conductivity is higher than the low-frequency conductivity then the rock is assumed to be oil-wet. However, this result is merely qualitative. Bona, N., Rossi and Capaccioli, S., 1999, “Wettability and Dielectric Constant”, Proceedings of the Annual Meeting of the Society of Core Analysts, SCA-9925 describes the response of glass bead samples and Berea sandstones with various wettability between 100 Hz and 100 MHz. Apparent differences in the water- versus oil-wet sample responses are noted. However, wettability effects are not decoupled from other critical parameters such as rock porosity, water saturation and water salinity.